1. Field of the Invention
The present invention relates to a solid-state imaging device such as a charge-coupled device (CCD) and, more particularly, to a solid-state imaging device having a high-speed shutter function.
2. Description of the Related Art
A solid-state imaging device such as a CCD is a comparatively recent development, and a color video camera incorporating this device is now commercially available. In this color video camera, the charge storage time of the photosensitive section of the device corresponds to the shutter time of a still photography camera, and is normally set to 1/60 sec.
In the meantime, however, a frame interline transfer (FIT) type CCD has been developed, which allows arbitrary selection of the shutter speed. A typical FIT type CCD comprises photosensitive and high-speed vertical transfer sections which constitute an imaging section, a field memory which serves as a storage section, and a horizontal transfer section. This type of CCD operates in the following manner:
(a) Charges stored in the photosensitive section are simultaneously transferred to the high-speed transfer section in response to a discharge field shift pulse supplied from a driver. A high-speed clock pulse is then supplied from the driver to the high-speed transfer section, and the charges are discharged at a high speed.
(b) Next, charges stored in the photosensitive section are simultaneously transferred to the high-speed transfer section in response to a signal read field shift pulse supplied from the driver, and are then transferred to the field memory in units of lines and at a high speed. Thereafter, the charges in the field memory are transferred to the horizontal transfer section, again in units of lines but at a normal television frequency, where they are then read within one horizontal period.
Thus, according to the conventional FIT type CCD, the charges in the photosensitive section are shifted in response to the discharge field shift pulse, after which the charges read in response to the signal read field shift pulse are stored in the storage section. Therefore, the time interval between the discharge and signal read field shift pulses constitutes the shutter speed. By selecting this time interval, a specified shutter speed can be set.
In the conventional video camera using the FIT type CCD, a one-field image can be obtained by a high-speed shutter operation. However, if a two-field (i.e., one-frame) image constituted by odd- and even-numbered fields is obtained so as to improve the vertical resolution, two shutter operations are required. If two shutter operations are performed in the above-described camera, the time interval between first and second shutter operations is 1/60 sec. Accordingly, although the one-field image can be obtained by the high-speed operation, the one-frame image corresponds to an image obtained by repeating the high-speed operation twice at a time interval of 1/60 sec. Therefore, when the motion of an object to be imaged is fast, flicker is caused and the resultant image becomes poor.